Light and Electron Microscopic Observations on the Early Ontogeny of the Adventitious Root in Glechoma hederacea L.

Abstract

Summary In vegetative material of Glechoma hederacea a minimum of two pairs of adventitious roots per node normally occurs on the plagiotropic shoot. At the oldest node of the terminal bud the first initiated pair of primordia is already apparent and the second pair is freqently present. At older nodes additional roots may develop and precocious branching of the root often occurs due to necrosis of the apex. This may already become evident whilst the root is still enclosed within the parent stem. In the orthotropic flowering shoots adventitious roots are normally absent. The provascular tissue of the stem is composed of four strands of procambium separated by tracts of residual meristem and the root is initiated endogenously from the latter tissue and the adjacent layer of cortical parenchyma cells which both undergo de-differentiation to the primary meristematic condition. The root primordium then enlarges and becomes organised into cap, ground meristem and provascular tissue, whilst the apical initials soon show conspicuous vacuolation, suggesting the development of a quiescent centre. No cytological evidence has been obtained to support the theory that breakdown of the stem cortex by the advancing root tip is effected by enzymatic secretion from this organ and it is probable that penetration occurs by mechanical pressure.